Spatial Light Modulators (SLM's) are widely used in the industry for video monitors, graphic displays, projectors, and hard copy printers. Spatial light modulators typically modulate incident light to form a light image. This light image is directed to a screen in the case of a projector, video monitor or display, or is ultimately focused on a light sensitive material, such as a photoreceptor drum, in the case of a xerographic printer.
A recent innovation of Texas Instruments Incorporated of Dallas, Tex. is the digital micromirror device (DMD). The DMD is a spatial light modulator suitable for use in displays, projectors and hard copy printers. The DMD is a micromechanical monolithic single-chip integrated circuit, comprised of a high density area array of 16 micron square deflectable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of memory cells and address electrodes, these memory cells being controlled by a row address circuit and loaded/unloaded by column pixel data shift registers. Each mirror forms one pixel of the DMD array, and is bistable through electrostatic attraction forces, that is to say, stable in one of two deflected positions. A source of light is directed upon the mirror array, and is reflected in one of two directions by each mirror. In one stable "on" mirror position, incident light is reflected by the mirror to a collector lens and focused on the display screen, or directed to the photoreceptor drum, and comprises one pixel. In the other "off" mirror position, light directed on the mirror is deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the collector lens, or, to the light absorber. The collector lens may be used in combination with a light prism to ultimately focus and magnify the light image from the pixel mirrors when projected onto a display screen to produce a viewable image. If each pixel mirror of the DMD array is in the "on" position, the light image will be a bright array of pixels.
For a more detailed discussion of the DMD device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference.
Image display systems, such as those incorporating the DMD device, require that the image projected on the display screen be in focus, for both rear-screen and front-projection display systems. In some conventional auto-focusing systems, an infrared-type range-finder system is employed to determine the distance from the projector lens to the image plane, i.e. the screen, and adjust the focal length of the projector lens based on the ascertained distance. Such a system is relatively expensive and subject to some discrepancies.
The present invention sets forth to provide an inexpensive auto focus system for an image display system, and particularly, for an image display system including a micromechanical SLM such as the DMD having an array of micromechanical pixels with well defined geometric physical features.